1. Field of the Invention
The present invention relates to, inter alia, an electric rotating machine, and more specifically relates to an electric rotating machine preferably used as an electric motor as a driving source for various electric vehicles including electric motorcycles and various electric machines.
2. Description of the Related Art
The following description sets forth related art and problems therein and should not be construed as an admission of knowledge in the prior art.
Conventionally, electric vehicles, such as, e.g., electric motorcycles, are equipped with an electric motor as a driving source. Further, various electric devices, such as, e.g., DVDs, are also equipped with an electric motor as a driving source. An electric rotating machine such as an electric motor includes a rotor and a stator. The rotor includes a permanent magnet and is configured to rotate about a rotation axis. The stator includes stator windings and is arranged so as to face the rotor via a gap in a radial direction of the rotor.
In recent years, it has been desired that an electric motor used as a driving source of various electric vehicles including electric motorcycles may be small in size and high in performance. In the case of a vehicle equipped with an internal combustion engine, a transmission is normally used. In the case of electric motors of this kind, however, if the operational range from a high torque low speed revolution speed range to a low torque high speed revolution speed range is wide, a driving force appropriate for a vehicle operation can be obtained without using a transmission.
In an electric motor, however, due to the inherent characteristics, a high torque can be generated in a low revolution speed range. In an electric motor, however, the upper limit of the revolution speed will be limited in a high revolution speed range. That is, in an electric motor, a high torque can be generated in a low revolution speed range. However, as the revolution speed increases, the induced voltage (e.g., back electromotive force), which is to be generated at the stator winding arranged on the stator by magnetic flux of the permanent magnet provided at the rotor, increases. When the revolution speed increases and reaches a certain speed, the induced voltage induced at the stator winding becomes equal to the applied voltage of the electric motor, preventing the electric current flow in the stator winding. This in turn prevents a further increase of the revolution speed. To solve this problem, for example, a field weakening control, which decreases the induced voltage (e.g., back electromotive force), is employed.
In the field weakening control, a current to negate the induced voltage is supplied to the stator winding. In the case of an electric motor in which an electric power is supplied from the outside, the increased power consumption does not result in a shortened drivable time. Therefore, in such an electric motor, even if such a field weakening control is employed, there will be no problem. However, in the case of an electric motorcycle, no power is supplied thereto from the outside, and the electric motorcycle is driven only by a power supplied from a battery mounted thereon. In such a vehicle, the battery capacity is limited. Therefore, when the electric current is supplied to negate the induced voltage induced in the stator winding, the electric power consumption increases. This results in a shortened drivable time. For this reason, it is requested to decrease the power consumption as much as possible.
Herein is proposed a new stator structure capable of being replaced with a conventional field weakening control which induces additional power consumption (see Japanese Unexamined Laid-open Patent Application Publication No. 2006-191782). In this proposal, a tooth portion of a stator on which a winding is arranged is divided into at least two divided tooth portions. The at least two divided tooth portions are relatively movable. The relative movement of the at least two divided tooth portions causes magnetic flux flow changes, which decreases the influence on the stator winding by the flux of the permanent magnet of the rotor at the time of a high revolution speed. According to this proposal, the flux linkage of the stator winding at the time of a high revolution speed can be adjusted by a physical means. Therefore, the electrical power conventionally used for the field weakening control can be decreased or eliminated, which enabled to provide an electric rotating machine capable of decreasing power consumption.
In such an electric rotating machine of this type, it is desired to further enlarge the operational range from a high torque low revolution speed range to a low torque high revolution speed range.